The object of the present invention is a process for simultaneously measuring the propagation characteristics of a plurality of radio-frequency channels. It finds an application in radio-communications with mobiles. The radio-frequency channels, whose characteristics are being measured, are here wideband.
In a radio-communications system with mobiles, as for example in the GSM system or in the IS-95 system, a microwave signal is propagated between fixed base stations, connected to the telephone network, and mobile users, to be called hereinafter mobile stations. Each connection between a base station and a mobile station is made by means of a radio-frequency or mobile radio channel.
The microwave signal reaches the mobile station (in a so-called down direction) or the base station (in a so-called up direction) with more or less attenuated and phase offset echoes, a consequence of the obstacles encountered during propagation. The signal undergoes overall an attenuation which fluctuates according to the displacements of the mobile. Two fluctuation scales may be distinguished: rapid fluctuations, caused by interference, and slow fluctuations due to modifications in the environment or in mobile station-base station separation when the mobile is displaced over great distances. These attenuations affect in the first place the signal-to-noise ratio. But multiple paths also generate interference between symbols, which is all the more marked the higher the information rate.
To improve transmission performance, it is conceivable to transmit the same signal from several different sources (transmitter diversity) or to receive it in different places (receiver diversity). When one of the connections becomes too attenuated or is subject to too much interference between symbols, an automatic transfer (or xe2x80x9chandoverxe2x80x9d) procedure is used. Processing algorithms are also conceivable which combine the signals coming from several channels.
A growing number of high bit rate transmission systems provide for the use of space diversity or polarisation diversity. Space micro-diversity (using several close transmit and receive antennae) or polarisation micro-diversity (dual polarisation transmission or reception) make it possible to counter multiple paths and rapid fading. Macro-diversity (connection between one mobile and several base stations) may also be used to this end. It also makes it possible to counter masking effects and to smooth transitions between cells.
It consequently becomes indispensable to have full control over measuring the propagation characteristics of the different channels which may be used in diversity so that new networks can be designed and deployed in the best way.
Measurement cycles in micro-diversity at mobile station level or in polarisation diversity at mobile station level do not raise any particular problems. The channel sensing signal is transmitted from the base station. The antennae are placed on a vehicle containing the reception means, these antennae being a few centimetres apart in space micro-diversity or superimposed in polarisation diversity. They are connected to a same sensing device, which will for example read alternately on one and on the other the signal delivered by the antennae. Rotating the vehicle wheel triggers the signal acquisitions. It is said that the configuration is xe2x80x9cmaster distancexe2x80x9d It is easy to recognise the exact position of each measuring point when going through the results.
Micro-diversity at base station level and macro-diversity are much trickier to achieve. These difficulties may be stressed by distinguishing xe2x80x9cmaster timexe2x80x9d methods and xe2x80x9cmaster distancexe2x80x9d methods:
a) xe2x80x9cMaster Timexe2x80x9d Methods
In space micro-diversity at the base station, several antennae are a few centimetres apart. In polarisation diversity, they are superimposed.
The channel between the mobile station and the base station being reciprocal, the signal may very well be transmitted from the mobile station and be received in diversity at the base station. A receive end technician must always be present to trigger the beginning then the end of the measurements. Acquisitions occur at a regular rate; they are controlled by a receive-end clock. The method is called xe2x80x9cmaster timexe2x80x9d.
In this configuration, the vehicle must travel at a constant speed so that the exact location of the measuring points may be reconstituted. This is only possible over small sections and some routes may have to be ruled out because of road traffic.
In macro-diversity, the antennae are spaced apart by several hundred metres. Channel measurements in xe2x80x9cmaster timexe2x80x9d therefore require several sensing devices, in the event one at each station. A technician must be present on each site. The problems previously raised in respect of micro-diversity remain. To these may be added difficulties in synchronising measurements: the moments of triggering and then stopping the recordings must be common to the different sites.
b) xe2x80x9cMaster Distancexe2x80x9d Methods
Different methods have been proposed for carrying out measurements in diversity at the base station or in macro-diversity, on two channels, and in a configuration of the xe2x80x9cmaster distancexe2x80x9d type.
A first solution, the most straightforward conceptually, consists in transmitting the same sensing signal on the two connections. Superimposition of the impulse responses of the two channels is obtained. These responses are disassociated if they are sufficiently offset in time. For this the transmitter sequences must be synchronised. This technology is described in the article by M. G. KADEL entitled xe2x80x9cMeasurement of wideband micro-and macro-diversity characteristics of the mobile radio channelxe2x80x9d published in IEEE, Proc. of VTC, Stockholm, Sweden, 1994, pages 165 to 169.
Synchronisation of sequences is a priori possible in micro-diversity or in polarisation diversity. It is complex to achieve in macro-diversity since two independent transmitters are being worked with. The sequences can always be randomly offset by reinitialising a sequence of one or other of the transmitters. At all events, good transmitter synchronisation is only verified at the receive end by displaying the main impulse response peaks and by noting their effective separation. It is only on this condition that measurements of a section may begin.
The length of a sequence must be at least equal to twice the spread of delays of the impulse responses measured. In macro-diversity, the peaks of the two impulse responses shift independently of each other during displacement of the mobile. It is therefore necessary for there to be ample room for manoeuvre, especially when synchronisation is obtained randomly by reinitialising a transmitter.
The sequence length used in practice with this method is of the order of 100 xcexcs. The method is difficult to apply to more than two transmitters, since recognition then synchronisation of sequences may prove difficult to achieve.
Another problem appears in macro-diversity since the oscillators of the two transmitters drift apart independently. They are not subject to the same clock and the impulse responses calculated may in the end overlap. The results become unusable when these drifts are substantial, which may unfortunately occur while it is difficult to verify stability at the time of measurement.
A second solution is described in the article by G. KADEL entitled xe2x80x9cSimulation of the DECT System Using Wideband Channel Data Measured in Two Diversity Branches, Proceedings of the 2nd International Conference on Universal Personal Communications (ICUPC), Ottawa, pp. 546 to 550. This solution consists in offsetting the carrier frequency of the second transmitter relative to that of the first. In practice, an offset xcex94f of about 20 Hertz may be introduced. The composite signal received is demodulated to the frequency of the first transmitter. After processing, the two superimposed impulse responses may be observed. The slight frequency offset of the second transmitter introduces artificially a Doppler effect. The measurement remains of good quality since the sequence is of very short duration, at the most 100 xcexcs, comparatively to the 50 ms of the Doppler period. When the mobile is in sleep mode, the impulse responses corresponding to the second channel fluctuate in time, whereas those of the first do not vary. Two ways of conducting the experiments are conceivable:
for each measuring point, the mobile stops then initiates a series of recordings; after processing, the Doppler spectrum of each delay is calculated in baseband from the successive responses; the contribution to this spectrum of the channel 1 is theoretically of one null frequency line, that of the channel 2 of a line distant by xcex94f from the first; the sampling theorem is satisfied when the acquisitions have occurrences of 25 ms; low pass filtering in baseband insulates the information from one or the other channel;
when the mobile is moving very slowly, it may be supposed that the artificial Doppler effect takes precedence over the real Doppler effect; it is then conceivable for the acquisitions to be made during displacement and always at regular intervals of time; the contribution of each channel to the Doppler spectrum of a given delay is no longer a simple line; it remains however centred around this line; as a precaution, in practice the acquisitions must be lower than 25 ms; to avoid a two channel frequency overlap, the speed of the mobile is limited to 1 m/s (4 km/h).
This method too has numerous drawbacks. Firstly, it does not offer a practical xe2x80x9cmaster distancexe2x80x9d solution. It compels, indeed, stopping at each measuring point in the first version, or travelling very slowly (4 km/h) at uniform speed in the second. Moreover, channel separation leads to sluggishness in the calculations. Each impulse response sample is obtained by filtering the information observed over about a hundred consecutive acquisitions at a given delay. This operation must be repeated for each sample of the sequence received, therefore a good hundred times. The complexity of receive-end processing is multiplied by a factor of 10,000 at least relative to a diversity-free measuring method.
When measurements are triggered every 25 ms, the time required for the acquisition of about a hundred sequences takes more than two seconds. The channel must therefore be presupposed to be stationary over this period of time. Such a hypothesis may well not be verified when vehicles are moving about in the vicinity of the mobile.
The purpose of the present invention is indeed to overcome these drawbacks of the prior art. It proposes a process of the xe2x80x9cmaster distancexe2x80x9d type allowing the simultaneous analysis of several channels on a same band, while combining ease of implementation with signal processing simplicity. The invention is implemented so as to observe the impulse responses over a time duration T adapted to the environment to be sensed.
The process of the invention does not require the sequence of one of the transmitters to be reinitialised or the offsets between sequences to be followed over time. It operates in real time; a measurement at a certain moment makes it possible to calculate the impulse responses of the channels at that moment. It is very economical in calculation time; its complexity is that of diversity-free measurement techniques, multiplied by the number of channels.
The measurements being controlled in distance, the vehicle is no longer compelled to move at a uniform speed. Reconstituting routes therefore does not pose any major problems. It is no longer necessary to mobilise a technician to trigger the measurements on each site. The synchronisation problems raised in respect of macro-diversity are also avoided.
In an exact way, an object of the present invention is a simultaneous measurement process, over an observation period T, of the propagation characteristics of a plurality of p radio-frequency channels (p whole number at least equal to 2) between p base stations and a mobile station, characterised in that:
from each of the base stations is transmitted simultaneously a periodic signal of period pT, the signal transmitted by the station of rank n (n going from 1 to p) having a spectrum constituted by lines located on frequencies (k/T)+((nxe2x88x921)/pT) where k is the number of the line,
the p signals transmitted by the p stations are received simultaneously in the mobile station and the signals received are processed in a time window of width pT so as to extract the p impulse responses of the p radio-frequency channels.
To transmit from the nth station the signals in question, the following procedure may be adopted:
a sequence of elements of duration T is produced,
this sequence is reproduced with a period T so as to obtain a periodic signal of period T,
a carrier wave is produced having a frequency equal to the central frequency Fc of the frequency band to be analysed,
the frequency of the carrier wave is offset by the quantity (nxe2x88x921)/pT,
the signal obtained is transmitted after modulation of the carrier thus offset.
Preferably, the band of the transmitted signals is reduced by filtering so as to limit the band to a width B, the spectra of the transmitted signals extending from xe2x88x92B/2 (inclusively) to B/2 (exclusively) and the number k thus taking all the whole values between xe2x88x92N (inclusive) and Nxe2x88x921 (inclusive) where N is equal to BT/2 (which will be supposed whole).
To obtain the signals to be transmitted by the base stations, memories may also be read containing appropriate samples of the signals having the spectra in question.
As far as processing carried out in the mobile station is concerned, preferably, the signal received is sampled at a sampling frequency at least equal to the width B of the band used for transmission.
To obtain the impulse response of each channel, the following procedure may be adopted:
a) a determination is made of:
the amplitude of the lines located on the
frequencies k/T to obtain a first spectrum,
the amplitude of the lines on the frequencies k/T +(nxe2x88x921)/pT to obtain an nth spectrum,
the amplitude of the lines on the frequencies k/T +(pxe2x88x921)/pT to obtain a pth spectrum,
b) for each spectrum, is calculated the ratio of the amplitude of its lines to the amplitude of the corresponding lines of the transmitted signal,
c) an inverse Fourier transform of the different ratios obtained is carried out.
A different possible procedure would be to correlate the signal received, on a window of time duration pT, with the different signals transmitted by the first, through the nth, through the pth base station, so as to obtain the impulse response of the first, through the nth, through the pth channel.
The process which has just been defined is applied whatever the number of channels. In the particular case where this number is equal to 2, the process is such that:
the signal transmitted by one of the stations has a line spectrum on the frequencies k/T,
the signal transmitted by the other station has a line spectrum on the frequencies (k/T)+(xc2xdT),
the signal received by the mobile station is processed in a window of width 2T.
FIG. 1 shows diagrammatically a radio-communications system with several base stations and one mobile station;
FIGS. 2A and 2B show a bit sequence and the corresponding spectrum after filtering;
FIGS. 3A and 3B show a received sequence and the corresponding spectrum after filtering;
FIG. 4 shows the pace of an impulse response;
FIG. 5 shows an example of bit sequences forming a periodic signal;
FIG. 6A shows a first carrier wave and FIG. 6B a second carrier wave obtained from the first by frequency offset of xc2xdT;
FIG. 7A shows the spectrum S1(f) of a first signal transmitted by a first base station and FIG. 7B the line spectrum S2(f) of a second signal transmitted by a second base station;
FIG. 8 shows a first embodiment of the means making it possible to generate the two signals transmitted by the base stations;
FIG. 9 shows a second embodiment of the means making it possible to generate the two signals transmitted by the base stations;
FIGS. 10A and 10B show the influence of a window of width 2T applied to a signal;
FIGS. 11A, 11B, 11C and 11D show different signals appearing in frequency signal processing;
FIG. 12 shows an example of two impulse responses in the situation where the base stations are visible from the mobile station;
FIG. 13 shows another example of two impulse responses, in the situation where the base stations are not visible from the mobile station.